Modern motor vehicles commonly sport multicoat color and/or effect paint systems. Generally speaking, these multicoat paint systems comprise an electrocoat, a surfacer coat, anti-stonechip primer or functional coat, a color and/or effect basecoat, and a clearcoat. The multicoat paint systems are produced preferably by means of what are called wet-on-wet processes, in which a clearcoat film is applied to a dried, uncured basecoat film, and then at least basecoat film and clearcoat film are jointly cured thermally. This process may also be extended to include the production of the electrocoat and the surfacer coat, anti-stonechip primer or functional coat.
In these systems, the surfacer coats, anti-stonechip primers or functional coats are critical for such essential technological properties as impact resistance and smoothness and leveling of the overall finish. As a consequence, the requirements imposed on the quality of the surfacer coats, anti-stonechip primers or functional coats are particularly exacting. The systems must also be able to be produced easily and with outstanding reproducibility.
The automobile industry is concerned, moreover, to reduce the dry film thicknesses of the surfacer coats, anti-stonechip primers or functional coats, in order to lower the costs of raw materials and energy, without this being accompanied by any deterioration in the profile of performance properties of the multicoat paint systems, and particularly no deterioration in UV stability.
Important contributions towards solving these problems have been provided by the processes known from patent applications DE 44 38 504 A1, WO 2005/021168 A1 and WO 2006/062666 A1. The processes coat a substrate with an electrocoat material. The resulting electrocoat film is baked. The electrocoat is coated with a first, physically or thermally curable, aqueous basecoat material. The resulting first basecoat film, without being fully cured beforehand, is coated with a second, thermally curable, aqueous basecoat material. The resulting second basecoat film, without being fully cured beforehand, is coated with a clearcoat material, to produce a clearcoat film. Subsequently the first and second basecoat films and the clearcoat film are jointly baked.
The first, physically or thermally curable, aqueous basecoat material comprises as a binder at least one water-dilutable polyurethane resin, especially acrylated polyurethanes. Components of the first basecoat material may include titanium dioxide as pigment, talc as filler, and UV absorbers. The first basecoat material produces a first basecoat or functional coat, which at dry film thicknesses <35 μm, preferably of about 15 μm, is able to replace the conventional surfacer coats, anti-stonechip primers or functional coats without a loss of key technological properties of the multicoat paint systems. Moreover, the use of UV absorbers, especially UV-absorbing pigments, as described in WO 2005/021168 A1 and WO 2006/062666 A1, ensures that the UV stability of the multicoat paint systems in question is secured.
Where the above-described multicoat paint systems are exposed to stone chipping, there are instances, in spite of their high stonechip resistance, of flaking of the overall coating system, and in such cases the bare metallic substrate is exposed and is subjected to attack by corrosion. This corrosion is manifested in the formation of blisters, which are bubblelike eruptions in the multicoat paint system, accompanied by progressive enlargement of the area exposed by the stone chipping, as a result of the corrosive undermining of the multicoat paint system starting from the corrosion on the bare metallic substrate.
There is therefore a need to develop coating compositions for multicoat paint systems that protect the bare metallic substrate, exposed by impact load, by means of corrosion inhibitors which are already present in the coat system. In this context it is necessary for the corrosion inhibitors to have on the one hand a sufficiently high mobility to reach the exposed metallic substrate and on the other hand to be incorporated effectively in the coat system, in order to prevent unnecessary bleeding in humidity cycles as a result of osmotic pressure.
The corrosion inhibitors that are customarily used in the electrocoat film are pigmentlike and are added with the binder. Low molecular mass corrosion inhibitors can only reach the interface between substrate and paint, and hence be deposited, in the deposition process when they carry a positive charge; corrosion inhibitors of this kind usually have an adverse effect on the properties of the overall paint tank and hence of the finish. However, the particle size of pigmentlike corrosion inhibitors means that they have very little mobility or none at all.
DE 103 00 751 A1 describes coating compositions which can comprise up to 5% by weight, based on the coating composition, of water and/or solvents, and which in accordance with the invention are intended for the direct coating of metals, more particularly for the coating of metal strips, but which may also be applied over an electrocoat film. The coating compositions are cured with actinic radiation and comprise low molecular mass organic corrosion inhibitors and, preferably, further inorganic anticorrosion pigments. Besides the corrosion inhibitors and/or anticorrosion pigments, there may additionally be color pigments present in the coating composition. A multicoat paint system in automotive OEM finishing, as outlined in the introduction, is not described.
Where an electrocoat film is coated, more particularly over electrocoat films in automotive OEM finishing, using a coating composition which is cured with actinic radiation, the electrocoat film is sensitively damaged by photodegradation, leading to significantly reduced adhesion of the electrocoat film and hence to increased corrosive undermining of the coat in the vicinity of the bare metallic substrate—this phenomenon being what the present invention is specifically intended to avoid. Moreover, the application properties of the coating compositions described in DE 103 00 751 A1 can be adapted only with high cost and complexity to the application conditions, particularly with regard to the rheology, of the kind that are necessary for the above-described multicoat paint systems in automotive OEM finishing.